The neutralizing antibody response to Influenza A virus is thought to be specific for a given viral subtype. There are 16 influenza A subtypes defined by their hemagglutinins (HA). The 16 HAs, H1-H16, can be classified into two groups. Group 1 consists of H1, H2, H5, H6, H8, H9, H11, H12, H13, and H16 subtypes, and group 2 includes H3, H4, H7, H10, H14 and H15 subtypes. While all subtypes are present in birds, mostly H1, H2 and H3 subtypes cause disease in humans. H5, H7 and H9 subtypes are causing sporadic severe infections in humans and may generate a new pandemic. H1 and H3 viruses continuously evolve generating new variants, a phenomenon called antigenic drift. As a consequence, antibodies produced in response to past viruses are poorly or non protective against new drifted viruses. A consequence is that a new vaccine has to be produced every year against H1 and H3 viruses that are predicted to emerge, a process that is very costly as well as not always efficient. The same applies to the production of a H5 influenza vaccine. Indeed it is not clear whether the current H5 vaccines based on the Vietnam or Indonesia influenza A virus isolates will protect against a future pandemic H5 virus.
For these reasons it would be highly desirable to have a vaccine that induces broadly neutralizing antibodies capable of neutralizing all influenza A virus subtypes as well as their yearly variants (reviewed by Gerhard et al., 2006). In addition broadly neutralizing heterosubtypic antibodies could be used in preventive or therapeutic settings.
Antibodies that recognize influenza A virus have been characterized. Antibodies to M2 (an invariant small protein expressed on infected cells but not on infectious viruses) have shown some protective effect in vivo, possibly by targeting infected cells for destruction by NK cells or cytotoxic T cells. However, the HA is the primary target of neutralizing antibodies. It comprises a large ectodomain of ≈500 amino acids that is cleaved by host-derived enzymes to generate 2 polypeptides that remain linked by a disulfide bond. The larger N-terminal fragment (HA1, 320-330 amino acids) forms a membrane-distal globular domain that contains the receptor-binding site and most determinants recognized by virus-neutralizing antibodies. The smaller C-terminal portion (HA2, ≈180 amino acids) forms a stem-like structure that anchors the globular domain to the cellular or viral membrane. The degree of sequence homology between subtypes is smaller in the HA1 polypeptides (34%-59% homology between subtypes) than in the HA2 polypeptide (51%-80% homology). The most conserved region is the sequence around the cleavage site, particularly the HA2 N-terminal 11 amino acids, termed fusion peptide, which is conserved among all influenza A virus subtypes. Part of this region is exposed as a surface loop in the HA precursor molecule (HA0), but becomes inaccessible when HA0 is cleaved into HA1/HA2. In summary there are conserved regions among different HA subtypes especially in the HA1-HA2 joining region and in the HA2 region. However these regions may be poorly accessible to neutralizing antibodies.
There has only been limited success in identifying antibodies that neutralize more than one subtype of influenza A virus and their breath of neutralization is narrow and their potency is low. Okuno et al, (1993) immunized mice with influenza virus A/Okuda/57 (H2N2) and isolated a monoclonal antibody (C179) that binds to a conserved conformational epitope in HA2 and neutralizes the group 1 H2, H1 and H5 subtype influenza A viruses in vitro and in vivo in animal models (Okuno et al., 1993; Smirnov et al., 2000; Smirnov et al., 1999).
Recently Gioia et al., described the presence of H5N1 virus neutralizing antibodies in the serum of some individuals that received a conventional seasonal influenza vaccine (Gioia et al., 2008). The authors suggest that the neutralizing activity might be due to antibodies to neuraminidase (N1). However, monoclonal antibodies were not isolated and target epitopes were not characterized. It is not clear whether the serum antibodies neutralize other subtypes of influenza A virus.
Despite decades of research, there are no marketed antibodies that broadly neutralize or inhibit influenza A virus infection or attenuate disease caused by influenza A virus. Therefore, there is a need to identify new antibodies that protect again multiple subtypes of influenza A virus.